Sites like these are good especially if you are looking in a certain area; there are so many postings that it increases the odds that something will be posted for yours. However, the downside is that they utilize keywords, meaning that you will find every posting with a certain keyword in it. You will need to try a variety of keywords -- and also know that you will get "junk" that you aren't looking for at all. Again, these sites are good and should be used during your search, but don't make them your only source for job postings.
A list of some keywords to help you find appropriate jobs can be found at this link.